Coated abrasive grains and the method of coating the abrasive grains



Patented Feb. 13, 1951 COATED ABRASIVE GRAINS AND THE ltIETHOD OFCOATING THE ABBASIVE GRAINS Jacob S. Masin, Anniston, and Tull C. Allen,

ford, Ala., assignors to Monsanto Chemical Company, St. Louis, Mo., acorporation of Delaware No Drawing. Application November 4, 1947, SerialNo. 784,072

23 Claims. 1

The present invention is directed to a new and improved polishingabrasive grain and to a method of producing same.

An object of the invention is to provide an abrasive grain of improvedbonding properties.

Another object is to provide granules or abrasive grits with an unfused,unvitrified film or coating which presents a rough, irregular mattedsurface imparting greater tenacity to the adhesive bond between thegranules or grits and the paper or cloth backing or support.

An additional object is to provide granules or abrasive grits uniformlycoated with an inert or substantially inert material which is physicallyand/or chemically bonded to the surface of the grain by means of anunfused, unvitrified water resistant microscopic coating or film of ametallic metaphosphate.

A further object is to provide granules or abrasive grits with a waterresistant film or coating bonding finely divided pigments or polishingpowders to the surface of the abrasive material.

A still further object is to provide coated granules or abrasive gritsaffording strong capilarity to liquid or mobile adhesives which whenapplied will adhere with greater tenacity and efliciency to the granule,thereby yielding a finished article of greater durability and strengthand, therefore, greater utility as an abrasive.

Other objects and advantages of the present invention will be apparentto those skilled in the art as the description proceeds.

It is well known that abrasive grains are used for only a fraction ofthe time they might be efliciently employed in view of their toughness,hardness and sharpness. The reason for this is that abrasive grains asordinarilyproduced are characterized by a smooth glassy surface to whichadhesives do not adhere readily and consequently the abrasive grains arenot retained in place long enough to achieve maximum efiiciency ingrinding. This fact is strikingly illustrated upon ins ection of therefuse of grinding and polishing operations, which almost invariablycontains a relatively high percentage of detached abrasive grains whosesharp edges are substantialy unimpaired.

Heretofore, abrasive grains have been treated in various ways in aneffort to increase their.

bonding properties and thereby improve the 2 grinding efiiciency ofabrasive articles made therewith and while some measure of success hasbeen achieved, the results have not been entirely satisfactory.

For example, one method which has been proposed involves etching thesurface of the grains by means of acids or acid salts, but this methodis objectionable because theabove compounds have a selective action onthe matrix of the rain in which the crystals are. set and bring about amore rapid break-down of the grain during use.

Other methods of improving the adhesiveness of abrasive grains whichhave been advanced, comprise roughing the surface of the grain byattrition; providing the grains with a fused siliceous coating whichpresents rough edges and/or prominences extending outwardly from thesurface of the grain; applying a ceramic material to the surface of thegrains and heating to produce an unfused, unvitriiied porous foraminousfilm; coating the grains with sodium silicate carrying various inertmaterials; treating the grains with an aqueous solution of th ch10-rides, nitrates or sulfates of iron or aluminum followed by heating todecompose these salts and thereby leave a residue thereof on the surfaceof the grain; and precoating the grains with resins of various types.subject to the disadvantage of being troublesome, time-consuming andexpensive to carry out and in some instances fail to provide anysubstantial improvement in the adhesiveness of the abrasive grain.

We have discovered that the adhesion between abrasive grains and bondinmaterials is increased to a surprising degree by providing the surfaceof the grains with an insoluble or substantially insoluble metallicmetaphosphate coating carrying finely divided inert or substantiallyinert material. This coating is applied as an aqueous solution of thecorresponding mono-metallic -orthophosphate containing the inert orsubstantially inert material, and then the coated grain is heated to atemperature sufiicient to molecular-1y dehydrate the orthophosphate butbelow the temperature at which a substantial amount of fusion of thecoating takes place. As a preferred alternative to this procedure, theaqueous solution of the mono-metallic orthophosphate is initiallyapplied However, these methods are as a coating to the grain and thenthe inert material is adhesively bonded thereto by intimately anduniformly mixing the two materials together.

sium orthophosphate or a mixture of mono-,

sodium and mono-potassium orthophosphates in the proportions to yieldupon molecular dehydration a composition consisting of substantially 80%sodium hexametaphosphate and 20% potassium metaphosphate. However, itshould be .clearly understood that the invention is not restrictedthereto since any and all mono-metallic orthophosphates coming withinthe scope of the above definition are contemplated.

The above mono-metallic orthophosphates, particularly mono-aluminumorthophosphate, are rather unique. in that, notwithstanding the factthey are employed in the form of aqueous solutions of relatively highviscosity, they are characterized by their unusual ability to thoroughlyand uniformly wet the surface of abrasive grains. As to the inertmaterial, any finely divided substances which are unreactive orsubstantially unreactive with the above mono-metallic 'orthophosphatesat temperatures below the fusion point of the correspondin metaphosphatemay be employed. Illustrative examples of such materials are red-oxideof iron, chromic oxide, manganese dioxide, titanium oxide, Kentucky ballclay, -boron carbide, silicon carbide, diamond dust, crushed firebrick,feldspars and the like, but of these red oxide of iron is preferred.

\ The particle size of the finely divided inert material may be variedwidely so long as there is a substantial difference between it and theabrasive grain to be coated. In general, it is .desirable to confine theinert material to a particle size which falls within the range of fromto microns.

The improved grinding and polishing material prepared in accordance withthe present invention is composed of a mass of granules, each of whichcomprises a core of abrasive materials such as fused aluminum oxide,silicon carbide, corundum, emery or garnet, and an insoluble orsubstantially insoluble metallic metaphosphate coating which adhesivelybinds one or more of the above inert or substantially inert materials tothe surface of the grain. This coating, which is microscopic inthickness and is apparently physically and/or chemically bonded to thegrain, presents 'an unfused, unvitrified, rough irregular mattedsurface, which, in contrast to the glasslike smooth surfaces of theuntreated grain, en ables the bonding material to grip the grains firmlyand adhere tenaciously thereto, thus retaining the granules in servicein an abrasive structure for a much longer period of time.

As an illustration of the preferred method of carrying out the presentinvention, reference is made to the following specific example.

' EXAMPLE I v 350 lbs. of #46 aluminum ,oxide abrasive grain and 1400grams (3.08 lbs.) of an aoueous solu- 1.4 at C.) were mixed together ina cement mixer for 15 minutes, whereupon 1590 grams (3.5 lbs.) of finelydivided F8203 was introduced and the mixing continued for minutes. Theresulting product was dried by heating and then calcined in a gas firedrotary calciner which was heated to a temperature of approximately 700C.

"The calcination step effected molecular dehydration of thevmono-aluminum orthophosphate, thus converting this material into waterinsoluble aluminum metaphosphate which tenaciously bonded the FezOa tothe surface of the grain. a

The coated product was characterized by a bright red color, an unfused,unvitrified, rough irregular matted surface and by the following screenanalysis which compared favorably with the required screen testspecifications for polishing grain.

Screens Per Cent a ram on Series Screen ficamms +30 None +40 14 0-15 +4572 100 +50 13 0-40 :tfio 1 In addition to the above properties, it wasfound that the coated grain had markedly improved adhesive properties ascompared with untreated grain and that consequently grinding wheelsprepared therewith using resin bonds displayed 17% greater eficiencythan similar wheels prepared with uncoated grain.

EXAMPLE II perature of about 700 C. operation brought about moleculardehydration of the mono-aluminum phosphate to the correspondingmetaphosphate and thereby adhesively bonded the finely divided F6203 tothe surface of the grain.

The coated grain was characterized by a bright red color and an unfused,unvitrified, rough irregular matted surface which substantially improvedthe bonding properties of the grain. In addition, the product was alsocharacterized by having a coating which was substantially free of watersoluble salts since no turbidity was produced when a sample of thecoated grain was added to water and the resulting mixture vigorouslyagitated.

The following table further illustrates the various operating conditionsand materials which tion of mono-aluminum orthophosphate (sp. 3.indicated in the above examples.

Table I Abrasive Phosphate Inert Material calcination ConditionsProperties of Product 100 rams of aluminum 2 c. c. of an aqueous soluoxide abrasive grain #24 of Al (HqP a (Sp. grit. Gr. 1.3 at 25 0.). i

l c. c. of an aqueous soluams of aluminum 100 8 tion of Fe 01 F009: (SD.

oxide abrasive grain 20 nt. Gr. 1.35 at 25 Do 4 c. c.of20%aqueoussolution of Mg(H1PO4)i. Do 2 c. c. of 30% aqueous solutionof Plastic Phosphate."

1.5 c. c. of aqueous solution of Al(H2P04ga (Sp. Gr. 1.35 at 25 C.

100 grams of silicon carbide abrasive grain.

1.5 grams of Kentucky Ball Clay.

1.0 gram of Fe,O;

1.5 grams of Felon--.

Heated for 20 min- Dark gray colored granules having an utes at 550 0.

unfused, unvitrified rough irregular matted surface. Granules did notproduce turbidity when added to water.

Bright red granules having a. water resistant unfused, unvitrificd,rough Heated for 30 minutes at 550 C.

irregular matted surface. Heated in muffle fur- Do.

nace to 700 C. Heated for 30 min- Do.

'utes at 550 0.

do Do.

It will be noted that in all instances the coated grain displayed anunfused, unvitrified rough irregular matted surface which was resistantto the action of water, thus indicating that the Plastic Phosphate hadbeen rendered substantially water insoluble and also that the aboveorthophosphates had been molecularly dehydrated to the metaphosphateform.

The expression "Plastic Phosphate used in the above table covers amixture of 80% sodium hexametaphosphate and 20% potassium metaphosphate.It is distinguished from the other coating materials in that it need notbe formed in situ but may be applied directly to the abrasive grain.However, it may be formed on the grain by employing equivalent amountsof the corresponding mixture of monosodium and monopotassiumorthophosphates and then heating the latter to the metaphosphate formingtemperature. Therefore, itis understood that the present inventioncovers both of these procedures.

" The various conditions of operation will now be discussed in greaterdetail.

The present method of improving the bonding properties of abrasivegrains has been applied to grains varying in particle size from 8 to 240mesh, but it is to be understood that the invention is not restricted tothis size range since particle size is not a critical variable providedthe grain is substantially larger than the inert matc-rial to be bondedthereto.

The bonding material, that is, the Plastic Phosphate and thewatersoluble mono-metallic orthophosphate, is preferably employed in anamount just sufiicient to coat-al1 the particles of the abrasive grainwith a thin film of the corresponding metaphosphate. This factor willvary with the phosphate, the thickness of the coating and the size ofthe grain. In general, however, from about 0.5% toabout by weight of thebondin phosphate is sufficient to give satisfactory grain coverage.Within the foregoing range, about 1.6% by weight is preferred.

The inert or substantially inert material to be adhesively bonded to thesurface of the grain by the phosphate coating is preferably employed inan amount varying from about 0.5% to about having a specific gravity at25 C. of from 1.2 to 1.7 and within this range a specific gravity ofl.35-1.5 is preferred. I

The drying operation is generally carried out with agitation at 100 C.to 200 C., but higher or lower temperature may be used if desired.

The calcination step iscarried out ata temperature suflicient tomolecularly dehydrate the mono-metallic orthophosphate to metaphosphatebut below that temperature at which a substantial amount of fusion ofthe metaphosphate takes place. Since the limits of the temperature rangevary with the mono-metallic orthophosphate selected and moreover sincethis information is either available in the literature or is readilyascertainable by one skilled in the art, no attempt will be made to morespecifically define the calcination temperature range. In general,however, a temperature of about 550 C. is preferred. fl

With reference to the use of PlasticPhosphate, it is essential tocalcine the coating, whether directly applied to the abrasive grain orformed in situ, so as to convert the metaphosphates into a.substantially water insoluble form.

This is preferably accomplished by heating the 2% by weight, but largeror smaller amounts may coated product for about 30 minutes at atemperature of about 550 C.

Where inert material is referred to in the claims, it is to beunderstood that this expression likewise covers substantially inertmaterials such as iron oxide, chromium oxide, manganese oxide andequivalent materials.

.While we have described our invention in detail, it should beunderstood that many changes may be made therein without departing fromspirit of same.

What we claim is:

1. Abrasive grain coated with a substantially water insoluble, unfused,unvitrified film which bonds finely divided inert materials to thesurface of the grain, said film consisting essentially of a materialselected from the group consisting of aluminum metaphosphate, ferricmetaphos-- .phate, magnesium metaphosphate and Plastic Phosphate.

2, Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrifled film which bonds finely divided inertmaterials to the surface of the grain, said film consisting essentiallyof a material selected from the group consisting of aluminummetaphosphate, ferric metaphosphate, magnesium metaphosphate and PlasticPhosphate. V

3. Silicon carbide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds finely divided inertmaterials to the surface of the grain, said film consisting grain, saidfilm consisting essentially of "Plastic essentially of a'materialselected from the group consisting of. aluminum metaphosphate, ferricmetaphosphate, magnesium metaphosphate and Plastic Phosphate.

4. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds from 0.5% to 2% byweight of finely divided inert materials to the surface of the rain,said film consisting essentially of a material selected from'the groupconsisting of aluminum metaphosphate, ferric metaphosphate, magnesiummetaphosphate and Plastic Phosphate. 1 r

5. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused. unvitrinum metapliosphate.

'1 5.,.The method 'of 'improving the bonding properties oif'-abras'iye"grains, which comprises -coatin'gthem" w itha materialconsisting essen- "tiallyof af solution of a mono-metallicorthophosphatecarrying finely divided inert material -:;and .-thenheating" the coated grain to a temper- )a'ture' sufficientq tomolecularly dehydrate said 'ortlio'phos'phate to metaphosphate but belowthe fied film which bonds from 0.5% to 2% by weight of finely dividedferric oxide to the surface of the.

grain, said film consisting essentially'of azmaterial selected from thegroup consisting of aluminum metaphosphate, ferric metaphosphate,magnesium metaphosphate and "Plastic'Phosphate."

6. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused; unvitrified film which bonds from 0.5% to 2% byweight of finely divided clay to the surface of the grain, said filmconsisting essentially of a material se lected from the group consistingof aluminum metaphosphate, ferric metaphosphate, magnesium metaphosphateand Plastic Phosphate.

7. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds about 1% by weight offinely divided inert materials to the surface of the grain said filmconsisting essentially of a material selected from the groupconsisting'of aluminum metaphosphate, ferric metaphosphate, magnesiummetaphosphate and "Plastic Phosphate.

temperature at which substantial fusion of the -metapliosphate takesplace, said mono-metallic orthophosphate being selected from the group8. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds about 1.5% by weight offinely divided ferric oxide to the surface of the grain, said filmconsisting essentially of aluminum metaphosphate.

9. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds about 1% by weight offinely divided ferric oxide to the surface of the grain, said filmconsisting essentially of aluminum metaphosphate.

10. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds about 1.5% by weight offinely divided clay to the surface of the grain, said film consistingessentially of aluminum metaphosphate.

11. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, un'fused, unvitrified film which bonds about 1.4% by weightof finely divided ferric oxide to the surface of the grain, said filmconsisting essentially of ferric metaphosphate.

12. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds about 1% by weight offinely divided ferric oxide to the surface of the grain, said filmconsisting essentially of magnesium metaphosphate.

13. Aluminum oxide abrasive grain coated with a substantially waterinsoluble, unfused, unvitrified film which bonds about 1% by weight offinely divided ferric oxide to the surface of the consisting ofmono-alummum orthophosphate. mono-iron orthophosphate', mono-magnesiumortnopnosphate and a mixture or mono-potassiumand mono-sodiumorthophosphates in amounts equivalent to "Plastic Phosphate.

16. The method of improving the bonding properties of aluminum oxideabrasive grains, which comprises coating said grains with a materialconsisting essentially of a solution of a mono-metallic ortho-phosphate,applying finely divided ferric oxide to the coated grains and heatingtne resulting product to a temperature sufiicientto molecularlydehydrate said orthophosphate to metaphosphate but below the temperatureat which substantial fusion of the metaphosphate takes place, saidmono-metallic orthophosphate being selected from the group consisting ofmono-alummum orthopnosphate,

mono-nod orthophosphate, mono-magnesium oithophosphate and a mixture ofmono-potassium and mono-sodium orthophosphates in amounts equivalent to"Plastic Phosphate.

17. The method of improving the bonding properties of aluminum oxideabrasive grains, which comprises coating said grains with a materialconsisting essentially of a solution of mono-aluminum ortnophosphate,applyingfinely divided ferric oxide to the coated grains and heating theresulting product to a temperature sufiicient to molecularly dehydratesaid orthophosphate to metaphosphate but below the temperature at whichsubstantial fusion of the metaphosphate takes place.

18. The method of improving the bonding properties of aluminum oxideabrasive grains, which comprises coating said grains with a materialconsisting essentially of a solution of monoiron orthophosphate,applying finely divided ferric oxide to the coated grains and heatingthe resulting product to a temperature suflicient to molecularlydehydrate said orthophosphate to metaphosphate but below the temperatureat which substantial fusion of the metaphosphate aqueous solution ofmono-metallic orthophosphate has a specific gravity of 1.2 to 1.7 at 25C.

21. The method defined in claim 16 wherein the aqueous solution ofmono-metallic orthophosphate has a specific gravity of 1.35 to 1.50 at25 C.

22. The method defined in claim 16 wherein an aqueous solution ofmono-aluminum orthopnosphate having a specific gravity of about 1.4 at25 C. is employed.

23. The method of improving the bonding properties of silicon carbideabrasive grains, which comprises coating said grains with an aqueoussolution of mono-aluminum orthophosphate having a specific gravity of1.35 at 25 C.. applying finely divided ferric oxide to the coated grainsand heating the resulting product to a temperature sufiicient tomolecularly dehydrate said orthophosphate to metaphosphate but below thetemperature at which substantial fusion of the metaphosphate takesplace.

JACOB S. MASIN. TULL C. ALLEN.

10 REFERENCES CITED The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 930,376 Higgins Aug. 10, 1909944,436 Higgins et al Dec. 28, 1909 1,001,572 Allen Aug. 22, 19111,061,255 Allen et a1 May 13, 1913 1,400,495 Anderson Dec. 13, 19211,482,792 Hartmann Feb. 5, 1924 1,482,793 Hartmann Feb. 5, 19241,549,409 Gravell Aug. 11, 1925 1,910,444 Nicholson May 23, 19331,987,861 Milligan et a1 Jan. 15, 1935 2,123,419 Gund1ack July 12, 19382,281,167 Nichols Apr. 28, 1942 2,303,284 Klein Nov. 24, 1942

1. ABRASIVE GRAIN COATED WITH A SUBSTANTIALLY WATER INSOLUBLE, UNFUSED,UNVITRIFIED FILM WHICH BONDS FINELY DIVIDED INERT MATERIALS TO THESURFACE OF THE GRAIN, SAID FILM CONSISTING ESSENTIALLY OF A MATERIALSELECTED FROM THE GROUP CONSISTING OF ALUMINUM METAPHOSPHATE, FERRICMETAPHOSPHATE, MAGNESIUM METAPHOSPHATE AND "PLASTIC PHOSPHATE"
 15. THEMETHOD OF IMPROVING THE BONDING PROPERTIES OF ABRASIVE GRAINS, WHICHCOMPRISES COATING THEM WITH A MATERIAL CONSISTING ESSENTIALLY OF ASOLUTION OF A MONO-METALLIC ORTHOPHOSPHORATE CARRYING FINELY DIVIDEDINERT MATERIAL AND THEN HEATING THE COATED GRAIN TO A TEMPERATURESUFFICIENT TO MOLECULARLY DEHYDRATE SAID ORTHOPHOSPHATE TO METAPHOSPHATEBUT BELOW THE TEMPERATURE AT WHICH SUBSTANTIAL FUSION OF THEMETAPHOSPHATE TAKES PLACE, SAID MONO-METALLIC ORTHOPHOSPHATE BEINGSELECTED FROM THE GROUP CONSISTING OF MONO-ALUMINUM ORTHOPHONSPHATE,MONO-IRON ORTHOPHOSPHATE, MONO-MAGNESIUM ORTHOPOSPHATE AND A MIXTURE OFMONO-POTASSIUM AND MONO-SODIUM ORTHOPHOSPHATES IN AMOUNTS EQUIVALENT TO"PLASTIC PHOSPHATE."